To answer your question, there are two types of maps stored on your phone: those which you downloaded by going to Settings > Swipe to Applications > Maps > Download maps > then choose whatever map to download, and the maps which are automatically downloaded onto your phone when you launch your map app. You can delete the first type in the same place (Settings > Swipe to Applications > Maps > Download maps > select the maps you want to delete*. For the second type which have been automatically downloaded: Settings > Swipe > Maps > Delete History*.

But I'm not sure what you're referring to as "Phone Storage" amounting to 1.2GB. If your maps have been downloaded onto the SD Card, deleting them will not substantially affect your phone storage.

*- You can also access this from Storage Sense > phone > maps > manage >.